1. Field of the Invention
This invention relates to adhesive compositions having improved adhesive properties, in particular improved resistance to cathodic disbonding and to the use of such adhesives in bonding protective coatings to substrates.
To protect steel, or other metal, pipes and pipelines from corrosion, a protective coating is generally applied. The protective coating can be, for example, a polymeric sheet, tape or sleeve of, for example polyethylene, bonded to the surface of the pipe. Various adhesives and mastics can be used to bond the coating to the pipe. However, accidental damage to the protective coating can occur during installation and use of the pipe or pipeline exposing some of the surface of the pipe to the environment.
Corrosion of a steel or iron pipe when exposed to the environment is due to electrochemical reactions in which the pipe loses iron ions to water in the environment coating localized anodic regions on the pipe. To prevent the development of these anodic regions and thus the attendent corrosion of the pipe, an electric current can be impressed along the length of the pipe. The pipe is connected to a negative electrical potential, thus causing the pipe to act as a cathode. Further loss of iron ions to the environment is inhibited.
It is known that adhesive compositions generally used to bond a protective coating to a pipe are adversely affected by this imposed electrical current. The result of this is a weakening of the adhesive bond between the coating and the pipe such that the coating comes away from the pipe. More of the surface of the pipe is then exposed to the corrosive conditions of the environment. This phenomenon is referred to in the art as "cathodic disbonding".
In commonly assigned U.S. patent application Ser. No. 92,852, filed Nov. 9, 1979 a polyhydroxy compound is added to an adhesive composition to improve its resistance to cathodic disbondment. It has now been discovered that the addition of a hydrazine derivative to an adhesive composition improves the resistance to cathodic disbonding.